1. Field of the Invention
The present invention is related to a position touch screen panel and method of arranging the resistive sensing circuit, and more particularly to an analog-based touch screen panel that is used as an input to electronic devices.
2. Description of Related Art
Touch screen panels can be driven by resistive, capacitive, ultrasonic or infrared mechanism. Since the costs of resistive components are more competitive than the others, resistive touch screen panels are widely used as inputs to PDAs, electronic notepads, LCD display terminals, etc.
A position touch screen panel is basically formed by two glass substrates symmetrically overlapped, leaving a narrow gap in between the substrates. A conductive film is coated on the underside of the substrates, and a sensing circuit is arranged on the periphery of the substrates. When an object or finger touches the surface of the substrate, the resistive sensing circuit is excited to produce a corresponding voltage gradient, and thus the coordinates of the point of contact can be determined.
In this line of products, analog-based touch screen panels are the mainstream, as digital-based touch screen panels at this stage are not yet cost competitive. The costs of related components need to be further reduced to allow for large-scale production.
In FIG. 4, a position touch screen panel is formed by two symmetrically overlapped substrates (71) (72), each having a conductive layer on the inner surface, and a sensing circuit consisting of four sensor lines (Xin) (Xout) (Yin) (Yout) is arranged along the periphery of the substrates (71) (72).
The detailed arrangements of the sensor lines (Xin) (Xout) (Yin) and (Yout) are to be described below, in conjunction with FIGS. 5A, 5B.
The two sensor lines (Xin) (Xout) on the substrate (71) are used to measure a voltage gradient in the X direction, whereas the two sensor lines (Yin) (Yout) on the substrate (72) are used to measure a voltage gradient in the Y direction.
The X direction sensor line (Xin), as shown in FIG. 5A, is formed on the right side along the periphery of the substrate (71) connecting corner A and corner B serially, whilst sensor line (Xout) is formed on the left side of the substrate (71), connecting corner D and corner C serially. The input and output terminals are located on one end, in the middle section on the lower side of the substrate (71).
The Y direction sensor line (Yin), as shown in FIG. 5B, is formed on the lower side of the substrate (72) along the periphery, and an input terminal is connected to the middle section of the sensor line (Yin). The sensor line (Yout) starts off from one end, which is the output terminal, on the lower side of the substrate (72) and adjacent to the terminal end of the sensor line (Yin), and bends to the left and runs along the lower side to corner D, and then bends upward to corner C, and then again bends to the right and runs along the upper side to corner B of the substrate (72).
When an object or finger touches the surface of the substrate, a voltage excitation is produced (5 V is used in the present example). The measured voltage from corner A connection of the sensor line (Xin), as shown in FIG. 5A, is approximately 4.9718V. As the sensor line extends, the internal impedance of the line causes decay of signal strength in proportion to the length of the sensor line, so that the measured voltage from corner B connection shall be lower, and in the present example, the voltage over corner B connection is approximately 4.9127V. Thus, voltage over corner A connection is defined as a high voltage level, whereas voltage over corner B connection is defined as a low voltage level, hereinafter respectively denoted by (Xin_High) and (Xin_Low). Voltage levels at corner points are defined relative to other corner points on one sensor line. Then, 0.0816V is measured over corner C connection of sensor line (Xout) and 0.0271V over corner D connection of sensor line (Xout), hereinafter denoted by (Xout_High) and (Xout_Low).
Referring back to FIG. 5B, the highest voltage (5V) appears in the middle section of the sensor line (Yin), near the input terminal, and the voltages over two corners A and D connections are almost equal (4.975V), thus these two are denoted by (Yin_Low). On another sensor line (Yout), the highest voltage is measured over corner B connection (0.797V), and the voltage over corner C connection is lower at (0.459V), hereinafter respectively denoted by (Yout_High) and (Yout_Low).
Since the two substrates (71) (72) are overlapped symmetrically, corners A–D on substrates (71) (72) are lined up to form parallel connection pairs. but this has created a problem of mismatch of voltage pairs.
Using corners A and C as an example to illustrate a mismatch of voltage level existing over the connection pairs in opposite corners of a conventional touch screen panel. The parallel connection pair in corner A is (Xin_High)+(Yin_Low), and the parallel connection pair in corner C is (Xout_High)+(Yout_Low), thus a (High+High) pair and a (Low+Low) pair in corner A and corner C of the touch screen panel cannot be created. According to the voltage levels previously defined for all corners, this circuit arrangement cannot attain matching voltages in opposite corners. Therefore, the sensing circuit cannot measure voltage gradient accurately during a touch of the screen, and the calculation of coordinates of contact point will produce substantial errors. For the same reason, the voltage levels in opposite corners B and D are also not symmetrical. The touch screen panel is therefore unable to determine touch positions accurately using the conventional pattern of arranging the sensing circuit.